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The mesomeric (resonance) effect is a permanent polarization transmitted through conjugated pi systems. Unlike the inductive effect, it does NOT diminish along the conjugation chain, making it the dominant electronic effect in unsaturated systems. Groups with +M effect (electron-donating: NH2 > OH > OR > F, Cl, Br) donate lone pairs into the pi system, enriching ortho and para positions. Groups with -M effect (electron-withdrawing: NO2 > CN > CHO > COR > COOH) withdraw electron density through pi conjugation. A crucial JEE concept is the behavior of halogens: they are -I (deactivating overall) but +M $\frac{directing to ortho}{para}$. The -I wins for activation/deactivation, but +M controls regiochemistry. Resonance structures must maintain the same atomic connectivity, total electron count, and formal charge. More equivalent structures provide greater stabilization — this is why the carboxylate anion (2 equivalent structures, charge on 2 oxygens) is more stable than phenoxide (5 structures, but charge on less electronegative carbons). When mesomeric and inductive effects oppose each other, mesomeric usually dominates in conjugated systems. This explains why -OCH3 is net electron-donating on benzene (activating, o/p director) despite oxygen's electronegativity — the +M effect through the pi system overrides the -I through sigma bonds.